PPO activity

A novel approach to analyzing inhibitory and noninhibitory compounds was adopted in an attempt to predict which structures might inhibit PPO. As a result, stilbenes with hydroxyl groups in their structure were found to inhibit PPO activity (Lopez-Nicolas and Garcia Carmona 2008). 

One of the most important applications of PPO, although rarely reported, is its role in synthetic processes, such as the biosynthesis of betalains. Several researchers reported the hydroxylation of tyrosine to dopa, which can then be oxidized to dopaquinone, through a PPO from Portulaca grandiflora and from Beta vulgaris. Thus, a dioxygenase activity complements the constitutive PPO activity and the initiation of this dioxygenase... 

For example, the resistance of plants to the pathogen Pseudomonas syringae was studied by Thipyapong and others (2004) in tomato plants into which antisense PPO cDNA was inserted. Their results showed a strong reduction of PPO activity and a dramatic increase in the susceptibility of plants, although the overall growth and development of the tomato plants was not affected by the downregulation of PPO. 

Figure 5. Ozone alteration of PAL and PPO activity in soybean leaves (8). Each mean is based on eight observations.

PPO catalyses the dependent oxidation of phenolics to quinones. The secondary reactions of quinones lead to the formation of polymeric brown or black pigments, which are responsible for significant post-harvest losses of fruits and vegetables [72]. Finally, induced PPO activity consists of both systemic and localized components. Systemic induction of PPO in tissues in response to all types of injuries may represent a broad, defensive role for PPO in protection of juvenile tissues from subsequent attack by a broad spectrum of pathogens and pests [71]. 

Phenolic accumulation in B-deficient tissues can activate a group of enzymes that use these compounds as substrates. In B-deficient sunflower leaves, PPO activity progressively augments as symptoms characteristic of this deficiency appear. Plants with sensitivity to B deficiency differ in PPO activity. The accumulation of phenolics and PPO activity are reportedly greater in more sensitive plants, such as sunflower, than in less deficiency-sensitive plants, such as com (Zea mays) [125]. 

As commented above, most works that relate B to phenol metabolism centre on the effect of B deficiency. Nevertheless, other works show the effect of growing dosages of the micronutrient on the metabolism of these secondary compounds. For example, Fawzia et al. [120] found that as the B dosage increased to 500 ppm, the phenolic concentration declined, as did the PAL and PPO activity, while the contrary effect was evident in POD activity. The high POD activity, together with the low PAL activity perhaps decreased the phenolic content. These same authors found... 

Influence defense response. Isozymes of these enzymes may also be numerous [as for PO (21)] and may be differentially Induced, not affected by pathogen action (S2), or Identical In resistant and susceptible cultivars. Since viruses do not have PO and PPO activity, these pathogens may be useful to evaluate the roles of those enzymes In primary Infection processes (SI). 

FIGURE 21.22 Impact of EDBM treatment on PPO activity in apple juice during storage. (Adapted from Lam Quoc, A., Lamarche, F., and Makhlouf, J., J. Agric. Food Chem., 48, 2160, 2000.)... 

The optimum pH for the activity of an enzyme preparation from any one source usually varies with different substrates and is characteristic of the substrate as well as the enzyme preparation. The strawberry PPO showed maximum activity at pH 5.5 with catechol, and at pH 4.5 with 4-meth-ylcatechol as substrate [72]. The type of buffer and the purity of the enzyme affect the optimum pH. Isoenzymes have also different pH optima Enzyme preparations obtained from the same fruit or vegetable at various stages of maturity have been reported to differ in optimum pH of activity as well [14], Very different pH optima have been reported for PPO activity ranging from acidic pH (3.5) in grape [73] to basic pH (9.0) in Amasya apple [74]. Two pH optima for PPO have also been reported in literature. The pH optima for the Jonagored apple PPO are 5.0 and 7.5 [75] and for the eggplant PPO, 4.0 and 6.5-7.S [71]. 

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